How to link custom object animation controls to rig components
I've found what looks like a good solution. Still need to test it in animation editing. In case anyone else is attempting to do this, this is what I've setup:


Create a new child bone of the knee bone (thigh bone, not the shin bone) and extrude it outward from the knee. Make sure this bone is "connected" to the parent. Call this bone IKKneePoleArm.L. Then extrude again (from this new bone) and call the second one IKKneePoleHandle.L. Disable Deform for both of these.
Setup normal IK using IKKneePoleHandle.L as the pole target for the leg.
Go into the bone settings and turn off inherit rotations for IKKneePoleArm.L. This will   keep the feedback problem (mostly) at bay.
Completely hide IKKneeHandle.L. You only need to rotate IKKneePoleArm.L around its orbit axis to aim the knee. You can also disable the other axes (in pose mode).
You can also attach a sphere object to the pole arm, so that it appears to be a sphere that orbits the knee. Then you can grab the sphere and push/pull it around and the system will convert that into rotations.


The only issue I've found with this so far is that some adjustments seem to lag behind by one frame by a small fraction. So if you type in values, you have to press enter twice (only if you are cursed as a perfectionist).

If there is a better solution anyone knows of, I'm definitely interested. Thanks!